In the motor application field, overload relays are used to protect the motor windings from over heating.
The principle of these overload relays consists of detecting the overload currents through the deflection of one or more bimetal strips due to the temperature rise provided by the current. The motor current either directly goes through the bimetal strip and or through a heater made of an appropriate resistant strip wound around it. The subassembly made of the bimetal strip, the heater and the insulating sheath is called a bimetal thermal element.
Relays designed for long tripping times are dedicated for some applications where the loads driven by the motors are huge and so the starting durations to get their full speeds are long.
Overload relay tripping times may refer to the classes defined within the international standards, for instance the standard IEC 60947-4. For example, if the tripping time under 7.2 In (In=nominal current) is between 6 and 20 s starting from a cold state, a relay is marked class 20.
The structure of a conventional bimetal thermal element 11 can be seen from FIG. 1. As shown in FIG. 1, it is made of the following components: a bimetal strip 14; a heater 15 made of a resistant strip wound around the bimetal strip 14, with several turns according to the electrical resistance to be obtained, and welded on the bimetal strip 14, and the end portion is connected to a terminal in the relay case (not shown in FIG. 1); an insulating sheath 16 to electrically insulate the heater 15 from the bimetal strip 14. The bimetal element 11 is assembled with a support 12 by different techniques such as riveting or laser welding and an input wire 13 connected with a motor is connected with the support 12.
As can be seen from above description, the current flows via the following path: from the input wire 13 to the bimetal support 12; from the bimetal support 12 to a first end of the bimetal strip 14 welded with the support 12; from the first end to a second end of the bimetal strip 14 opposite to the first end; from the second end of the bimetal strip 14 to the resistant strip 15 through the welding point between the resistant strip 15 and the bimetal strip 14; finally crossing all the turns of the resistant strip 15 around the bimetal strip 14 and reaching the welded to relay terminal. Therefore there is current in both the bimetal strip 14 and the bimetal support 12. For this configuration, since there is current in both the bimetal strip 14 and the bimetal support 12, the deflection of bimetal strip 14 is generated by the temperature-rise coming from the heater 15 as well as from the bimetal strip 14 and the bimetal support 12, and thus, it is hard to realize a long tripping time.
For the situation where a long tripping time is needed, usually a multi-strip heater made of several very thin and flexible strips in parallel welded together at both of their ends is used. This heater can be either in one short length or made of one or two go and return portions so as to increase the heater length. In the second case, the go and return lengths are insulated with thin insulating strips. One of the multi-strip heater ends is either welded on the bimetal strip or directly to the input depending on the current path desired. The other end is generally welded on the terminal of the relay. Finally the multi-strip heater is fixed along the bimetal strip by staples to ensure a good thermal intimacy. Between the heater and bimetal strip, another insulating strip is placed. But for this kind of heater, the thickness of the bimetal thermal element will increase since several layers of heater are used. In many situations, the thickness of the bimetal thermal element is limited to the size of the product casing, and such a multi-strip heater can not be fit into the casing.
Therefore, there exists the requirement to improve a bimetal thermal element to realize a longer tripping time while maintaining a small volume, especially a small width to fit for many applications.